1. Field of the Invention
This invention relates to a mis-convergence correction device for a color picture display tube using three electron beams. The mis-convergence correction device of this invention is connected to a deflection yoke of the color picture display tube when being used.
2. Description of the Related Art
In color picture display tubes using three electron beams, convergence means the condition in which the electron beams intersect at a specified point on a fluorescent screen. The color picture display tubes tend to have pincushion distortion which contracts a central area of the raster in upward and downward directions.
Some advanced deflection yokes of color picture display tubes are designed as precision convergence systems (PCS) which provide desired convergence and also suitably correct pincushion distortion.
The PCS deflection yoke positively distorts the distribution of generated magnetic field in the related color picture display tube so that the electron beams can accurately focus on a point of the screen. Thus, the PCS deflection yoke implements self-convergence.
The PCS deflection yoke is provided with a mis-convergence correction device connected to a pair of horizontal deflection coils. The horizontal deflection coils generate horizontal deflection magnetic fields, respectively. The mis-convergence correction device repetitively and differentially changes the horizontal magnetic fields at a vertical deflection period, thereby correcting mis-convergence along horizontal lines in upper and lower sides of the raster.
Japanese published unexamined patent application 5-328371 discloses a mis-convergence correction device which has a first controlled coil unit, a second controlled coil unit, and a controlling coil unit. The first controlled coil unit includes two drum cores formed with circular flanges at their ends. In the first controlled coil unit, two windings are provided on the drum cores respectively, and are connected in series to form a first controlled coil. The second controlled coil unit includes two drum cores formed with circular flanges at their ends. In the second controlled coil unit, two windings are provided on the drum cores respectively, and are connected in series to form a second controlled coil. The controlling coil unit includes a drum core formed with circular flanges at its ends. The controlling coil unit also includes a controlling coil provided on the drum core.
In the mis-convergence correction device of Japanese application 5-328371, the controlling coil unit is located between the first and second controlled coil units. The first and second controlled coils are connected to first and second horizontal deflection coils, respectively. The controlling coil is connected to a vertical deflection coil.
Japanese published unexamined patent application 7-23406 discloses a mis-convergence correction device which is similar to that of Japanese application 5-328371 except for the following point. In the mis-convergence correction device of Japanese application 7-23406, first and second controlled coil units, and a controlling coil unit are disposed in a casing composed of two halves.
A prior-art mis-convergence correction device which will be explained later has the following problems. The inductances of coils tend to vary from device to device. During assembly, adhesive is used. The use of the adhesive reduces the efficiency of assembly work.